1. Technical Field
The present invention relates to a sweetpotato expansin cDNA useful in the generation of highly productive transgenic plants. More particularly, the present invention relates to a sweetpotato expansin cDNA, a transformation vector carrying the same, and a method of preparing a transgenic plant having increased biomass and/or seed production.
2. Background Art
Since the discovery of expansin by Cosgrove and his colleagues (McQueen-Mason et al., 1992, Plant Cell 4, 1425-1433), intensive studies have been conducted thereon. In early studies, expansins were known as cell-wall-loosening enzymes that mediate, at least in part, pH-dependent extension of the plant cell wall and the growth of the cell (Cosgrove, 2000, Nature 407, 321-326). Since then, expansins were found to be in either α- or β-form (Shcherban et al., 1995, PNAS 92, 9245-9249). expansion, a variety of other plant processes, including morphogenesis (Ruan et al., 2001, Plant Cell 13, 47-60), softening of fruits (Rose et al., 2000, Plant Physiology 123, 1583-1592; Civello et al., 1999, Plant Physiology 121, 1273-1280), growth of the pollen tube (Cosgrove et al., 1997, PNAS 94, 6559-6564), elongation of graviresponding roots (Zhang and Hasenstein, 2000, Plant Cell Physiology 41, 1305-1312), and elongation of root cells (Lee et al., 2003, Plant Physiology 131, 985-997) (for review, Lee et al., 2001, Cur. Opin. Plant Biol. 4, 527-532).
Further, the expression pattern of expansins in flooded rice and tomatoes are well studied. It has been found that expansins are expressed in the shoot apical meristem of tomato for incipient leaf primordium initiation (Reinhardt et al., 1998, Plant Cell 10, 1427-1437). An expansin gene (Exp1) was cloned and found through transformants therewith to play an important role in the growth and ripening of tomato fruits (Brummell et al., 1999, Plant Cell, 11: 2203-2216). Expansin mRNA was accumulated just before the rate of growth or the loosening degree of the cell wall started to increase, suggesting that the expression of expansin genes is correlated with cell elongation (Cho and Kende, 1997a, Plant Cell 9, 1661-1671; 1997b, Plant Physiology 113, 1137-1143; 1998, Plant Journal 15, 805-812). Transgenic rice plants in which expansins are overexpressed were observed to further increase the length of cotyledons by 31-97% compared with the wildtype (Choi et al., 2003 Plant Cell, 15: 1386-1398). However, the transgenic rice plants are unable to bear seeds due to male sterility.
Increasing grain production is very important because seeds of grain plants are staple foods for most people. Since starch usually accounts for 60-70% of the weight of each grain, scientists have made extensive effort to augment the starch content of grains, thereby increasing grain production.
ADP-glucose pyrophosphylase (AGPase) is known as an allosteric enzyme which catalyzes the first committed step of starch synthesis in the plastid, converting glucose 1-phosphate and ATP to ADP-glucose and PPi and is exquisitely sensitive to allosteric regulation, with 3-phosphoglyceric acid (3PGA) acting as an activator and Pi as an inhibitor. Genes encoding this enzyme have been used in the study for enhancing starch synthesis in grains. A mutant (Sh2-Rev6) derived from maize AGPase by the addition of two amino acid residues (tyrosine and serine) corresponding to six nucleotides becomes insensitive to inhibitor of APGase and promotes starch synthesis in maize grains, increasing seed weight 11-18% (Giroux et al., 1996, PNAS 93, 5824-5829). The maize mutant AGPase (Sh2r6hs) has decreased sensitivity to the inhibitor and exhibits more stable binding between its small and large subunits. Transgenic Sh2r6hs wheat lines using a CaMV35S promoter produced on average 38% more seed weight per plant compared with wild-type (Smidansky et al., 2002, PNAS 99, 1724-1729). Transgenic Sh2r6hs rice lines using an endosperm-specific promoter (maize ubiquitin promoter) produced on average 20% more seed and plant weight compared with wild-type (Smidansky et al., 2003, Planta, 216, 656-664).
Therefore, there has been a need for transgenic plants that can remarkably increase biomass or seed production.